Defibrillators have long been used in the field of medicine to shock the heart into operating in a normal sinus rhythm when the heart goes into fibrillation. Defibrillators function by applying a relatively high powered pulse at a relatively high voltage between a pair of defibrillation electrodes that are placed against the chest of a patient. The relatively high power, high voltage characteristics of the defibrillation pulse require special electrodes that, among other things, provide a relatively large area of contact between the electrodes and the patient. For this reason, defibrillation electrodes are relatively expensive, typically selling to the end user at between $10 and $20 each.
It is often difficult to determine whether a patient is in need of defibrillation. An examination of the patient's electrocardiogram (ECG) is often helpful in making this determination. Consequently, many defibriilators include an ECG monitor and associated circuitry to obtain an ECG of the patient. The ECG monitor is normally connected to a pair of patient monitoring electrodes that adhere to the chest of the patient. The monitoring electrodes generate relatively low voltage, extremely low power, electrical signals indicative of the activity of the heart. The low voltage, low power characteristics of the signals generated by the monitoring electrodes, in contrast to the high voltage, high power pulses delivered to defibrillation electrodes, make monitoring electrodes substantially different from defibrillation electrodes. Monitoring electrodes typically have a relatively small area of contact with the patient, they are usually used once and then disposed of and they normally cost about 50:.
Monitoring electrodes are not suitable for use as defibrillation electrodes for several reason. For example, the relatively small contact area between the monitoring electrode and the patient would result in extremely high current densities if a defibrillation pulse was applied to the monitoring electrode. The high current density would severely burn the patient. Defibrillation electrodes are not normally used for patient monitoring because of their relatively high cost.
When an emergency health care practitioner responds to a cardiac emergency, he or she normally obtains the patient's ECG to determine if defibrillation is necessary. In many circumstances, defibrillation is not required. Thus, in most circumstances, a patient monitoring electrode is used, and the defibrillation electrodes are not required. If defibrillation is required, the health care practitioner must attach defibrillation electrodes to separate electrode leads or they must remove the patient monitoring electrode from the electrode leads and reconnect the defibrillation electrodes to the leads. Each of these procedures has a significant disadvantage.
The emergency conditions under which defibrillation normally occurs makes inadvertent defibrilation through patient monitoring electrodes quite possible. As mentioned above, the practitioner will initially connect the electrode leads to a patient monitoring electrode in order to obtain an ECG. If an emergency condition, e.g., fibrillation, then occurs, it is quite possible for the practitioner to forget that the defibrillator is connected to monitoring electrodes in the excitement of the emergency. In an effort to respond to the emergency as quickly as possible, the emergency health care practitioner may cause the defibrillator to generate a defibrillation pulse while the electrode leads are still connected to the monitoring electrode.
The danger of defibrillating through patient monitoring electrodes is even more acute because of the advent of automatic and semiautomatic defibrillators used by relatively untrained personnel. Most cardiac emergencies occur outside the presence of trained health care personnel. Recognizing that defibrillation must occur very shortly after the onset of fibrillation if it is to be successful, automatic and semiautomatic defibrillators have been proposed in order to allow even untrained personnel to defibrillate. In automatic or semiautomatic defibrillators, the patient's ECG is monitored and the defibrillator itself determines from the characteristics of the ECG whether defibrillation is required. In the automatic defibrillator, the defibrillator automatically generates a defibrillation pulse when defibrillation is required. In the semiautomatic model, the defibrillator informs the practitioner that defibrillation is required. The practitioner then manually triggers the defibrillator pulse. It is highly desirable in the case of a semiutomatic defibrillator, and absolutely required in the case of an automatic defibrillator for the defibrillator to determine whether it is connected to a defibrillation electrode. Obviously an automatic defibrillator should not generate a defibrillation pulse if it is connected to a monitoring electrode. While it is possible for an operator of a semiautomatic defibrillator to ensure that the defibrillator is connected to a defibrillation electrode prior to manually generating the defibrillation pulse, the relatively untrained nature of semiautomatic defibrillation operators makes the likelihood of mistake relatively high. Consequently, even for semiautomatic defibrillators, it is highly desirable for the defibrillator to be disabled from generating a defibrillation pulse if a patient monitoring electrode is connected to the defibrillator.
In addition to the problems resulting from attempting defibrillation through patient monitoring electrodes, problems also exist when defibrillation is attempted through improperly connected defibrillator or patient monitoring electrodes. If the electrode leads are short-circuited to each other when defibrillation is attempted, the relatively high current flow occurring through the short circuit can damage the defibrillator. More significantly, the short circuit prevents the defibrillation pulse from reaching the patient, thus making defibrillation impossible. Likewise, if the electrodes are not properly connected to the electrode leads, an open circuit condition can exist which also prevents the defibrillation pulses from reaching the electrodes. Consequently, automatic and semiautomatic defibrillators are not practical unless the defibrillator can determine for itself whether the defibrillator is properly connected to defibrillation electrodes.